Optical circulator and optical switch

ABSTRACT

Arranged, with spacing, are a first separating/combining birefringent element  10 , first and second optical-path controlling birefringent elements  11, 12 , and a second separating/combining birefringent element  13 . Between the first separating/combining birefringent element and the first optical-path controlling birefringent element, is arranged first polarization rotating means  14  for changing a polarization direction from an orthogonal to a parallel or from a parallel to an orthogonal. Also, between the second optical-path controlling birefringent element and the second separating/combining birefringent element, is arranged second polarization rotating means  17 . The first and second optical-path controlling birefringent elements are oppositely arranged symmetric in optical axis about an arrangement center thereof, between which is arranged a ½-wavelength plate for rotating 90 degrees a polarization direction of a light on a central optical path, thereby structuring an optical circulator. With a similar structure, an optical switch can be structured by using a ±45-degree variable Faraday rotator in the first and second polarization rotation switch means.

TECHNICAL FIELD

[0001] The present invention relates to an optical circulator for separating the light passing a transmission line or an optical switch for switching an optical path, in the field of optical communication, optical measurement and the like. More specifically, the present invention relates to an optical circulator and optical switch having a ½-wavelength plate provided at a center of an optical part arrangement to rotate 90 degree the polarization direction of a light on a central optical path, and the frontward and rearward, first and second optical-path controlling birefringent elements oppositely arranged such that the optical axes are symmetric about an arrangement center thereof. The typical example of the present invention is an optical circulator of a 4-port cyclic type or an optical switch of a 2×2 type.

BACKGROUND OF THE INNVENTION

[0002] The optical circulator is an optical device having a light separating function to output an input light from a certain port only to another particular port, e.g. output an input light from a first port P1 to a second port P2, and output an input light from a second port P2 to a third port P3. Meanwhile, the optical switch is an optical device having an optical-path switching function to output an input light from a first input port I1 to either a first output port O1 or a second output port O2, and output an input light from a second input port I2 to either a second output port O2 or a first output port O1.

[0003] Usually, the optical circulator uses a 45-degree Faraday rotator for applying a fixed magnetic field by a permanent magnet. By rotating the polarization plane 45 degrees in a predetermined direction, the non-reciprocality of light ray is realized. Although the optical switch realizes optical-path switching by the use of a variable Faraday rotator for changing the direction of an applied magnetic field due to an electromagnet, basic parts can be structured nearly in a similar manner.

[0004] Conventionally, there have developed various structures of optical circulators and optical switches. There is, as one example of those, a structure that four birefringent elements are arranged in one row with spacing, and sets of a Faraday rotator and a ½-wavelength plate are respectively inserted between the birefringent elements, to provide ports at both ends (e.g. see JP-A-2000-89164). Due to this, an optical circulator can be structured which couples an input light from a first port positioned at one end to a second port at the opposite end, an input light from a second port to a third port at its opposite end, an input light from a third port to a fourth port at its opposite end, and an input light from the fourth port to the first port at its opposite end, respectively.

[0005] In the above conventional structure, the ½-wavelength plate in a part (two plates positioned nearby the both ends) uses a combination, for each optical path, of four ½-wavelength plates different in optical-axis direction. Also, used in the arrangement center are ½-wavelength plates in combination arranged side by side with two-piece spacing. Thus, the structure is complicated and the parts are many in the number.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide an optical circulator and optical switch which can reduce the number of parts while maintaining a preferred characteristic.

[0007] According to a first aspect of the present invention, there is provided an optical circulator comprising:

[0008] a first separating/combining birefringent element for separating ones of light in an orthogonal relationship of polarization direction on a same optical path and combining ones of light on different optical paths,

[0009] first and second optical-path controlling birefringent elements for controlling an optical path depending on a polarization direction,

[0010] a second separating/combining birefringent element for separating ones of light on a same optical path and combining ones of light in an orthogonal relationship of polarization direction on different optical paths,

[0011] the first separating/combining birefringent element, the first and second optical-path controlling birefringent elements and the second separating/combining birefringent element being arranged in this order with spacing,

[0012] the first and second optical-path controlling birefringent elements being oppositely arranged such that optical axes are symmetric about an arrangement center thereof;

[0013] first polarization rotating means, for changing a polarization direction from an orthogonal to a parallel or from a parallel to an orthogonal, arranged between the first separating/combining birefringent element and the first optical-path controlling birefringent element,

[0014] second polarization rotating means, for changing a polarization direction from an orthogonal to a parallel or from a parallel to an orthogonal, arranged between the second optical-path controlling birefringent element and the second separating/combining birefringent element;

[0015] a ½-wavelength plate, for rotating 90 degrees a polarization direction of a light on a central optical path, arranged between the first optical-path controlling birefringent element and the second optical-path controlling birefringent element; and

[0016] ports arranged on a side of one of the separating/combining birefringent element and on a side of the other of the separating/combining birefringent element.

[0017] Herein, the first polarization rotating means and the second polarization rotating means may comprise a combination of a 45-degree Faraday rotator, a set of two ½-wavelength plates arranged side by side symmetrically in optical axes on the both optical paths to rotate a polarization direction 45 degrees, and a ½-wavelength plate for rotating 90 degrees a polarization direction of a light on a lower-staged optical path. These are arranged symmetric about a center of the arrangement.

[0018] Meanwhile, according to a second aspect of the invention, there is provided an optical switch comprising:

[0019] a separating birefringent element for separating ones of light in an orthogonal relationship of polarization direction on a same optical path,

[0020] first and second optical-path controlling birefringent elements for controlling an optical path depending on a polarization direction,

[0021] a combining birefringent element for combining ones of light in an orthogonal relationship of polarization direction on different optical paths,

[0022] the separating birefringent element, the first and second optical-path controlling birefringent elements and the combining birefringent element being arranged in this order with spacing,

[0023] the first and second optical-path controlling birefringent elements being oppositely arranged such that optical axes are symmetric about an arrangement center thereof;

[0024] first polarization rotating means having a ±45-degree variable Faraday rotator and for changing a polarization direction from an orthogonal to a parallel,

[0025] the first polarization rotating means being arranged between the separating/combining birefringent element and the first optical-path controlling birefringent element, second polarization rotating means having a +45-degree variable Faraday rotator and for changing a polarization direction from a parallel to an orthogonal,

[0026] the second polarization rotating means being arranged between the second optical-path controlling birefringent element and the combining birefringent element;

[0027] a ½-wavelength plate, for rotating 90 degrees a polarization direction of a light on a central optical path, arranged between the first optical-path controlling birefringent element and the second optical-path controlling birefringent element; and

[0028] input ports arranged on a side of the separating birefringent element and output ports arranged on a side of the combining birefringent element of the arrangement.

[0029] In the second aspect of the invention described above, the first polarization rotating means and the second polarization rotating means comprises, for example, a combination of a ±45-degree Faraday rotator, a set of two ½-wavelength plates arranged side by side symmetrically in optical axes on the both optical paths to rotate a polarization direction 45 degrees, and a ½-wavelength plate for rotating 90 degrees a polarization direction of a light on a lower-staged optical path. These are arranged symmetric about the arrangement center.

[0030] Although the wavelength plate serves to provide a phase difference of a predetermined wavelength to between orthogonal straight-line polarization components, the ½-wavelength plate in the present specification is to be assumably construed in a broad sense, assumably including those to cause a phase difference of (½+integer) respectively. Incidentally, the ½-wavelength plate, because of a nature to change a polarization direction of input light symmetrically about an optical axis thereof,.is a reciprocal optical parts capable of rotating a polarization plane of input light in a predetermined direction by previously setting an optical axis in a proper direction. Also, as for the ½-wavelength plate for insertion on a part of optical path, it is needless to say that the structure combined with a transparent member and adjusted same in outer shape as another optical part thereby facilitating assembling is included in the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is an explanatory view of an optical circulator according to an embodiment of the present invention.

[0032]FIG. 2A is a plan view showing an optical path of the optical circulator shown in FIG. 1.

[0033]FIG. 2B is a side view showing an optical path of the optical circulator shown in FIG. 1.

[0034]FIGS. 3A, 3B and 3C are explanatory views showing a direction of an optical axis of a ½-wavelength plate and 1/4-wavelength plate of the optical circulator shown in FIG. 1.

[0035]FIG. 4 is an explanatory view of an optical switch according to an embodiment of the present invention.

[0036]FIG. 5A is a plan view showing an optical path of the optical switch shown in FIG. 4.

[0037]FIG. 5B is a side view showing an optical path of the optical switch shown in FIG. 4.

BEST MODE FOR CARRYING OUT THE INVENTION

[0038]FIG. 1 is an explanatory view showing one embodiment of an optical circulator according to the present invention, while FIG. 2 is an explanatory view of an optical path of the same which is for a 4-port cyclic-type optical circulator. FIG. 1 shows an arrangement state of optical parts and a polarization state at a portion between the optical parts. Incidentally, the arrow in the optical part denotes an optical-axis direction or a Faraday-rotation direction. For the purpose of explanation, coordinate axes are set up as set forth in the following. The arrangement direction of the optical parts is taken as z-direction (depth direction on the drawing), while the two directions orthogonal thereto are taken as x-direction (horizontal direction on the drawing) and y-direction (vertical direction on the drawing). Also, concerning rotation direction, a clockwise direction as viewed in the z-direction is taken as a plus (+) side.

[0039] In the z-direction are arranged a first separating/combining birefringent element 10 for separating, in x-direction, the ones of light in an orthogonal relationship of polarization direction on the same optical path and for combining in the x-direction the ones of light on different optical paths, a first optical-path controlling birefringent element 11 allowing for normal light to travel straight and for abnormal light to shift an optical path toward +y-direction depending on a polarization direction, a second optical-path controlling birefringent element 12 allowing for normal light to travel straight and for abnormal light to shift an optical path toward −y-direction similarly depending on a polarization direction, and a second separating/combining birefringent element 13 for separating in the x-direction the ones of light in an orthogonal relationship of polarization direction on the same optical path and for combining in the x-direction the ones of light on different optical paths, in this order with spacing. Accordingly, the first optical-path controlling birefringent element 11 and the second optical-path controlling birefringent element 12 are oppositely arranged such that their optical axes are in symmetry about an arrangement center.

[0040] First polarization rotating means 14 for conversion of polarization direction from an orthogonal to a parallel (from a parallel to an orthogonal, in the reverse direction) is arranged between the first separating/combining birefringent element 10 and the first optical-path controlling birefringent element 11, as viewed in the z-direction. The first polarization rotating means 14 comprises a combination of a 45-degree Faraday rotator 15, a set of two ½-wavelength plates 16 arranged side by side such that the optical axes are in symmetry on the left/right both optical paths, and a ½-wavelength plate 17 acting for only the light on the lower-staged optical path to rotate its polarization direction by 90 degrees. Similarly, second polarization rotating means 18 for conversion of polarization direction from a parallel to an orthogonal (from an orthogonal to a parallel, in the reverse direction) is arranged between the second optical-path controlling birefringent element 12 and the second separating/combining birefringent element 13, as viewed in the z-direction. The second polarization rotating means 18 also comprises a combination of a ½-wavelength plate 19 acting for only the light on the lower-staged optical path to rotate its polarization direction by 90 degrees, a set of two ½-wavelength plates 20 arranged side by side such that the optical axes are in symmetry on the both left/right optical paths, and a 45-degree Faraday rotator 21.

[0041] Incidentally, the two 45-degree Faraday rotators 15, 21 are 45-degree fixed Faraday rotators using a permanent magnet and having a Faraday rotation angle of 45 degrees. Also, a set of two ½-wavelength plates 16, 20, as shown in FIG. 3A, has a left optical path having an optical axis inclining −22.5 degrees with respect to the x-direction and a right optical path having an optical axis inclining 22.5 degrees with respect to the x-direction, which are integrated into symmetry about the y-axis. The ½-wavelength plate 17, 19 has an optical axis inclining 45 degrees with respect to the x-direction, as shown in FIG. 3B.

[0042] Furthermore, viewing in the z-direction, a ½-wavelength plate 22 is provided between the first optical-path controlling birefringent element 11 and the second optical-path controlling birefringent element 12, which acts for only the light on a middle-staged optical path (central optical path in y-direction) to rotate the polarization direction thereof by 90 degrees. This ½-wavelength plate 22 has an optical axis also inclining 45 degrees with respect to the x-direction, as shown in FIG. 3C.

[0043] Then, viewing in the z-direction, on a side of the first separating/combining birefringent element 10, a first port P1 in the lower stage and a third port in the middle stage are set deviated in the y-direction while, on a side of the second separating/combining birefringent element 13, a second port P2 in the lower stage and a fourth port P4 in the middle stage are set deviated in the y-direction.

[0044] Now, explanation will be made on the operation of the optical circulator. FIG. 2A shows an optical path viewing in a −y-direction, while B shows an optical path viewing in a −x-direction.

[0045] (First Port P1→Second Port P2)

[0046] The light, inputted in the z-direction at the first port P1 in the lower stage, is allowed for normal light to travel straight in the first separating/combining birefringent element 10 and for abnormal light to be refracted and optically separated in the x-direction. In the first polarization rotating means 14, polarization direction is changed from an orthogonal into parallel relationship. Namely, the ones of light orthogonal in polarization direction are respectively rotated 45 degrees in polarization direction by the first 45-degree Faraday rotator 15 and inputted to the ½-wavelength plate 16. Because the ½-wavelength plate has a nature to symmetrically change the polarization direction of input light about its optical axis, the input ones of light rotate 45 degrees oppositely to each other into a vertical one to the x-axis. The both ones of light pass the ½-wavelength plate 17 positioned in the lower stage thereby being rotated 90 degrees in polarization direction into a parallel one with the x-axis. Accordingly, the both ones of light, because of normal light for the first optical-path controlling birefringent element 11, directly travels straight through an optical path in the lower stage. Because the central ½-wavelength plate 22 is inserted only in the middle-staged optical path, the light on the lower-staged optical path bypasses without undergoing any action. This light, because of normal light also for the second optical-path controlling birefringent element 12, travels straight the lower-staged optical path without change. Then, by the second polarization rotating means 18, the polarization direction is changed from a parallel into orthogonal relationship. Namely, the polarization direction is rotated 90 degrees by the ½-wavelength plate 19 into a vertical one to the x-axis, and polarization direction rotates 45 degrees respectively on the ½-wavelength plate 20 and the 45-degree Faraday rotator 21. The both ones of light become an orthogonal relationship. Then, because the light is combined in the x-direction by the second separating/combining birefringent element 13, it is outputted at the second port P2 in the lower stage.

[0047] (Second Port P2→Third Port P3)

[0048] The light, inputted in the −z-direction (reverse) at the second port P2 in the lower stage, is allowed for normal light to travel straight in the second separating/combining birefringent element 13 and for abnormal light to be refracted and optically separated in the x-direction. In the second polarization rotating means 18, polarization direction is changed from an orthogonal into parallel relationship. Namely, the ones of light orthogonal in polarization direction are rotated 45 degrees respectively in polarization direction by the second 45-degree Faraday rotator 21 and rotated 45 degrees in polarization direction oppositely to each other by the ½-wavelength plate 20 into a parallel one with the x-axis. The both ones of light pass the ½-wavelength plate 19 positioned in the lower stage, thereby being rotated in polarization direction to have a polarization direction vertical to the x-axis. The both ones of light, because of abnormal light for the second optical-path controlling birefringent element 12, are refracted upward to travel in the middle-staged optical path. By the central ½-wavelength plate 22, the polarization direction is rotated 90 degrees into a parallel one with the x-axis. This light, because of normal light for the first optical-path controlling birefringent element 11, travels, as it is, in the middle-staged optical path. Then, by the first polarization rotating means 14, the polarization direction is changed from a parallel to orthogonal relationship. Namely, bypassing the ½-wavelength plate 17, the rotation direction is rotated 45 degrees respectively by the ½-wavelength plate 16 and the 45-degree Faraday rotator 15. The both ones of light mutually become an orthogonal relationship. Then, because the light is combined in the x-direction by the first separating/combining birefringent element 10, it is outputted at the third port P3 in the middle stage.

[0049] (Third Port P3→Fourth Port P4)

[0050] The light, inputted in the z-direction at the third port P3 in the middle stage, is allowed for normal light to travel straight in the first separating/combining birefringent element 10 and for abnormal light to be refracted and optically separated in the x-direction. In the first polarization rotating means 14, polarization direction is changed from an orthogonal into parallel relationship. Namely, the ones of light orthogonal in polarization direction are respectively rotated 45 degrees in polarization direction by the first 45-degree Faraday rotator 15, and rotated 45 degrees oppositely to each other by the ½-wavelength plate 16, thus being placed in a normal one to the x-axis. The both ones of light bypass the ½-wavelength plate 17 positioned in the lower stage. Accordingly, because of abnormal light for the first optical-path controlling birefringent element 11, the light is refracted upward to travel in an optical path in the upper stage. Because the central ½-wavelength plate 22 is inserted only on the middle-staged optical path, the light on the upper-staged optical path bypasses without undergoing any action. This light, because of abnormal light for the second optical-path controlling birefringent element 12, is refracted downward this time to travel an optical path in the middle stage. Then, by the second polarization rotating means 18, the polarization direction is changed from a parallel to orthogonal relationship. Namely, bypassing the ½-wavelength plate 19, the polarization direction is rotated 45 degrees respectively by the ½-wavelength plate 20 and the 45-degree Faraday rotator 21. The both ones of light become an orthogonal relationship. Then, because the light is combined in the x-direction by the second separating/combining birefringent element 13, it is outputted at the fourth port P4 in the middle stage.

[0051] (Fourth Port P4→First Port P1)

[0052] The light, inputted in the −z-direction (reverse) at the fourth port P4 in the middle stage, is allowed for normal light to travel straight in the second separating/combining birefringent element 13 and for abnormal light to be refracted and optically separated in the x-direction. In the second polarization rotating means 18, polarization direction is changed from an orthogonal into parallel relationship. Namely, the ones of light orthogonal in polarization direction are rotated 45 degrees respectively in polarization direction by the second 45-degree Faraday rotator 21, and rotated 45 degrees in polarization direction oppositely to each other by the ½-wavelength plate 20 into a parallel one with the x-axis to bypass the ½-wavelength plate 19 positioned in the lower stage. Accordingly, the both ones of light, because of normal light for the second optical-path controlling birefringent element 12, travel straight as they are. By the central ½-wavelength plate 22, the polarization direction is rotated 90 degrees into a vertical one with the x-axis. This light, because of abnormal light for the first optical-path controlling birefringent element 11, is refracted downward to travel an optical path in the lower stage. Then, by the first polarization rotating means 14, the polarization direction is changed from a parallel to orthogonal relationship. Namely, by the ½-wavelength plate 17, the rotation direction is rotated 90 degrees. By the ½-wavelength plate 16 and the 45-degree Faraday rotator 15, the polarization direction is respectively rotated 45 degrees. The both ones of light become an orthogonal relationship. Then, because the light is combined in the x-direction by the first separating/combining birefringent element 10, it is outputted at the first port P1 in the lower stage.

[0053] In this manner, an optical circulator is realized which is of a 4-port cyclic type of from first port P1 to second port P2, from second port P2 to third port P3, from third port P3 to fourth port P4 and further from fourth port P4 to first port P1. Of course, unless the first port or fourth port is used, it can be used also as a 3-port type optical circulator without changing the structure.

[0054]FIG. 4 is an explanatory view showing one embodiment of an optical switch according to the present invention, while FIG. 5 is an explanatory view of an optical path thereof which is for a 2×2-type optical switch. FIG. 4 shows an arrangement state of optical parts and a polarization state at between the optical parts. Incidentally, the arrow in the optical part denotes an optical-axis direction or a Faraday-rotation direction. Also, in order to make the explanation easier to understand, coordinate axes are set up as in the above example of the optical circulator as follows. The arrangement direction of the optical parts is taken as z-direction (in a depth direction on the drawing), while the two directions orthogonal thereto are taken as x-direction (horizontal direction on the drawing) and y-direction (vertical direction on the drawing). Also, concerning rotation direction, a clockwise direction as viewed in the z-direction is taken as a plus side.

[0055] In the z-direction are arranged a separating birefringent element 30 for separating, in the x-direction, the ones of light in an orthogonal relationship of polarization direction on the same optical path, a first optical-path controlling birefringent element 31 allowing for normal light to travel straight and for abnormal light to shift an optical path toward +y-direction depending on a polarization direction, a second optical-path controlling birefringent element 32 allowing for normal light to travel straight and for abnormal light to shift an optical path toward −y-direction similarly depending on a polarization direction, and a combining birefringent element 33 for combining the ones of light in an orthogonal relationship of polarization direction on the optical paths different in the x-direction, in this order with spacing. Accordingly, the first optical-path controlling birefringent element 31 and the second optical-path controlling birefringent element 32 are oppositely arranged symmetrically in their optical axes about an arrangement center.

[0056] Viewing in the z-direction, first polarization rotating means 34 for conversion of polarization direction from an orthogonal to a parallel one is arranged between the separating birefringent element 30 and the first optical-path controlling birefringent element 31. The first polarization rotating means 34 comprises a combination of a first ±45-degree variable Faraday rotator 35, a set of two ½-wavelength plates 36 arranged side by side such that the optical axes are in symmetry on the left/right both optical paths, and a ½-wavelength plate 37 for acting only the light on the lower-staged optical path to rotate its polarization direction by 90 degrees. Similarly, second polarization rotating means 38 for conversion of polarization direction from a parallel to an orthogonal is arranged between the second optical-path controlling birefringent element 32 and the combining birefringent element 33. The second polarization rotating means 38 also comprises a combination of a ½-wavelength plate 39 for acting only the light on the lower-staged optical path to rotate its polarization direction by 90 degrees, a set of two ½-wavelength plates 40 arranged side by side such that the optical axes are in symmetry on the left/right both optical paths, and a second ±45-degree variable Faraday rotator 41. Incidentally, the two ±45-degree variable Faraday rotators 35, 41 are variable Faraday rotators of a structure to apply an external magnetic field to the Faraday element by the electromagnet, wherein the application magnetic field can be controlled in direction by switching the energizing direction to the electromagnet thereby making possible to switch Faraday rotation to ±45 degrees or −45 degrees. Herein, the both ±45-degree variable Faraday rotators 35, 41 are to be structurally switched together to have the same Faraday rotation direction. Also, the set of two ½-wavelength plates 36, 40, similarly to the showing in FIG. FIG. 3A, has a left optical path having an optical axis inclining −22.5 degrees with respect to the x-direction and a right optical path having an optical axis inclining 22.5 degrees with respect to the x-direction, which are mutually integrated symmetric about the y-axis. The ½-wavelength plate 37, 39 has an optical axis inclining 45 degrees with respect to the x-direction, similarly to the showing in FIG. 3B.

[0057] Furthermore, viewing in the z-direction, a ½-wavelength plate 42 is provided between the first optical-path controlling birefringent element 31 and the second optical-path controlling birefringent element 32, which acts for only the light in a middle-staged optical path (central optical path in the y-direction) to rotate its polarization direction by 90 degrees. This ½-wavelength plate 42 also inclines in optical axis with respect to the x-direction, similarly to the ½-wavelength plate shown in FIG. 3C.

[0058] Viewing in the z-direction, on a side of the separating birefringent element 30, a first input port I1 in the lower stage and a second input port in the middle stage are set deviated in the y-direction while, on a side of the combining birefringent element 33, a first output port O1 in the lower stage and a second output port O2 in the middle stage are set deviated in they-direction. Accordingly, this optical switch is of quite the same structure as the embodiment of optical circulator except for the structure of Faraday rotator.

[0059] Now, explanation will be made on the operation of the optical switch. FIG. 5A shows an optical path viewing in a −y-direction, while B shows an optical path viewing in a −x-direction.

[0060] First, an application magnetic field is set up in a direction (counterclockwise direction) that the Faraday rotation direction is minus as viewing the ±45-degree variable Faraday rotator 35, 41 in the z-direction.

[0061] (First Input Port I1→First Output Port O1)

[0062] The light, inputted in the z-direction at the first input port I1 in the lower stage, is allowed for normal light to travel straight in the separating birefringent element 30 and for abnormal light to be refracted and optically separated in the x-direction. In the first polarization switch means 34, polarization direction is changed from an orthogonal into a parallel relationship. Namely, the ones of light orthogonal in polarization direction are respectively rotated −45 degrees in the first ±45-degree variable Faraday rotator 35 and inputted to the ½-wavelength plate 36. Because the ½-wavelength plate has a nature to symmetrically change the polarization direction of input light about its optical axis, the input ones of light rotate 45 degrees oppositely to each other into a vertical one to the x-axis. The both ones of light pass the ½-wavelength plate 37 positioned in the lower stage thereby being rotated 90 degrees in polarization direction, whereby the polarization direction becomes parallel with the x-axis. Accordingly, the both ones of light, because of normal light for the first optical-path controlling birefringent element 31, directly travels straight through an optical path in the lower stage. Because the central ½-wavelength plate 42 is inserted only in the middle-staged optical path, the light on the lower-staged optical path bypasses without undergoing any action. This light, because of normal light also for the second optical-path controlling birefringent element 32, travels straight the lower-staged optical path without change. Then, by the second polarization rotating means 38, the polarization direction is changed from the parallel into an orthogonal relationship. Namely, the polarization direction is rotated 90 degrees by the ½-wavelength plate 39 into a vertical one to the x-axis, and polarization direction rotates 45 degrees respectively on the ½-wavelength plate 40 and the ±45-degree variable Faraday rotator 41. The both ones of light become an orthogonal relationship. Then, because the light is combined in the x-direction by the combining birefringent element 33, it is outputted at the first output port O1 in the lower stage.

[0063] (Second Input Port I2→Second Output Port O2)

[0064] The light, inputted in the z-direction at the second input port I2 in the middle stage, is allowed for normal light to travel straight in the separating birefringent element 30 and for abnormal light to be refracted and optically separated in the x-direction. In the first polarization switch means 34, polarization direction is changed from an orthogonal into a parallel relationship. Namely, the ones of light orthogonal in polarization direction are respectively rotated −45 degrees in the first ±45-degree variable Faraday rotator 35, and rotated 45 degrees oppositely with each other in the ½-wavelength plate 36, thus placed in a vertical one to the x-axis. The both ones of light bypass the ½-wavelength plate 37 positioned in the lower stage. Accordingly, because of abnormal light for the first optical-path controlling birefringent element 31, it refracts upward to travel in an optical path in upper stage. Because the central ½-wavelength plate 42 is inserted only in the middle-staged optical path, the light on the lower-staged optical path bypasses without undergoing any action. This light, because of abnormal light also for the second optical-path controlling birefringent element 32, is refracted downward this time to travel an optical path in middle stage. Then, by the second polarization rotating means 38, the polarization direction is changed from the parallel into an orthogonal relationship. Namely, the ½-wavelength plate 39 is bypassed, and polarization direction rotates 45 degrees respectively on the ½-wavelength plate 40 and the ±45-degree variable Faraday rotator 41. The both ones of light become an orthogonal relationship. Then, because the light is combined in the x-direction by the combining birefringent element 33, it is outputted at the second output port O2 in the middle stage.

[0065] Next, an application magnetic field is set up in a direction (clockwise direction) that the Faraday rotation direction is plus as viewing the ±45-degree variable Faraday rotator 35, 41 in the z-direction.

[0066] (First Input Port I1→Second Output Port O2)

[0067] The light, inputted in the z-direction at the first input port I1 in the lower stage, is allowed for normal light to travel straight in the separating birefringent element 30 and for abnormal light to be refracted and optically separated in the x-direction. In the first polarization switch means 34, polarization direction is changed from an orthogonal into a parallel relationship. Namely, the ones of light orthogonal in polarization direction are respectively rotated +45 degrees in the first ±45-degree variable Faraday rotator 35, and the input ones of light are rotated 45 degrees oppositely with each other in the ½-wavelength plate 36, thus placed in a vertical one to the x-axis. The both ones of light pass the ½-wavelength plate 37 positioned in the lower stage, whereby polarization direction rotates 90 degrees and the polarization direction is placed in a vertical one to the x-axis. Accordingly, because of abnormal light for the first optical-path controlling birefringent element 31, it refracts upward to travel in an optical path in middle stage. Then, by the central ½-wavelength plate 42, polarization direction is rotated 90 degrees and placed parallel with the x-axis. This light, because of normal light for the second optical-path controlling birefringent element 32, directly travels an optical path in middle stage. Then, by the second polarization rotating means 38, the polarization direction is changed from the parallel into an orthogonal relationship. Namely, the ½-wavelength plate 39 is bypassed, and the polarization direction rotates 45 degrees respectively on the ½-wavelength plate 40 and the ±45-degree variable Faraday rotator 41. The both ones of light become an orthogonal relationship. Then, because the light is combined in the x-direction by the combining birefringent element 33, it is outputted at the second output port O2 in the middle stage.

[0068] (Second Input Port I2→First Output Port O1)

[0069] The light, inputted in the z-direction at the second input port I2 in the middle stage, is allowed for normal light to travel straight in the separating birefringent element 30 and for abnormal light to be refracted and optically separated in the x-direction. In the first polarization switch means 34, polarization direction is changed from an orthogonal into a parallel relationship. Namely, the ones of light orthogonal in polarization direction are respectively rotated ±45 degrees in the first ±45-degree variable Faraday rotator 35, and rotated 45 degrees oppositely with each other in the ½-wavelength plate 36, thus placed in a parallel one to the x-axis. The both ones of light bypass the ½-wavelength plate 37 positioned in the lower stage. Accordingly, because of normal light for the first optical-path controlling birefringent element 31, it travels straight as it is. Then, by the central ½-wavelength plate 42, polarization direction is rotated 90 degrees and placed vertical to the x-axis. This light, because of abnormal light for the second optical-path controlling birefringent element 32, is refracted downward this time to travel an optical path in the lower stage. Then, by the second polarization rotating means 38, the polarization direction is changed from the parallel into an orthogonal relationship. Namely, the polarization direction rotates 90 degrees in the ½-wavelength plate 39, and the polarization direction rotates 45 degrees respectively on the ½-wavelength plate 40 and the ±45-degree variable Faraday rotator 41. The both ones of light become an orthogonal relationship. Then, because the light is combined in the x-direction by the combining birefringent element 33, it is outputted at the first output port O1 in the lower stage.

[0070] In this manner, an optical switch is realized which is of a 2×2 type of from first input port I1 to first output port O1, and from second input port I2 to second output port O2, or from first input port I1 to second output port O2 and from second input port I2 to first output port O1 by switching an application magnetic field. Of course, unless the ports in part are used, it can be used also as an optical switch of a 1×2 type or 2×1 type.

[0071] Incidentally, although not shown, in each of the above embodiments, the optical fibers to be coupled to the ports provided two on one side can be easily connected by using a two-cored ferrule.

[0072] The present invention, as in the above, has a structure having a ½-wavelength plate inserted on a central optical path in a center of an arrangement of optical parts, and first and second optical-path controlling birefringent element positioned forward and rearward thereof that are oppositely arranged such that the optical axes thereof are symmetric about the center of the arrangement. Accordingly, it is possible to obtain an optical circulator and optical switch that maintains preferred characteristics, having reduced number of ½-wavelength plates and easy to assemble. 

What is claimed is:
 1. An optical circulator comprising: a first separating/combining birefringent element for separating ones of light in an orthogonal relationship of polarization direction on a same optical path and combining ones of light on different optical paths, first and second optical-path controlling birefringent elements for controlling an optical path depending on a polarization direction, a second separating/combining birefringent element for separating ones of light on a same optical path and combining ones of light in an orthogonal relationship of polarization direction on different optical paths, the first separating/combining birefringent element, the first and second optical-path controlling birefringent elements and the second separating/combining birefringent element being arranged in this order with spacing, the first and second optical-path controlling birefringent elements being oppositely arranged such that optical axes are symmetric about an arrangement center thereof; first polarization rotating means, for changing a polarization direction from an orthogonal to a parallel or from a parallel to an orthogonal, arranged between the first separating/combining birefringent element and the first optical-path controlling birefringent element, second polarization rotating means, for changing a polarization direction from an orthogonal to a parallel or from a parallel to an orthogonal, arranged between the second optical-path controlling birefringent element and the second separating/combining birefringent element; a ½-wavelength plate, for rotating 90 degrees a polarization direction of a light on a central optical path, arranged between the first optical-path controlling birefringent element and the second optical-path controlling birefringent element; and ports arranged on a side of one of the separating/combining birefringent element and on a side of the other of the separating/combining birefringent element.
 2. An optical circulator according to claim 1, wherein the first polarization rotating means and the second polarization rotating means comprises a combination of a 45-degree Faraday rotator, a set of two ½-wavelength plates arranged side by side symmetrically in optical axes on the both optical paths to rotate a polarization direction 45 degrees, and a ½-wavelength plate for rotating 90 degrees a polarization direction of a light on a lower-staged optical path, these being arranged symmetric about a center of the arrangement.
 3. An optical switch comprising: a separating birefringent element for separating ones of light in an orthogonal relationship of polarization direction on a same optical path, first and second optical-path controlling birefringent elements for controlling an optical path depending on a polarization direction, a combining birefringent element for combining ones of light in an orthogonal relationship of polarization direction on different optical paths, the separating birefringent element, the first and second optical-path controlling birefringent elements and the combining birefringent element being arranged in this order with spacing, the first and second optical-path controlling birefringent elements being oppositely arranged such that optical axes are symmetric about an arrangement center thereof; first polarization rotating means having a ±45-degree variable Faraday rotator and for changing a polarization direction from an orthogonal to a parallel, the first polarization rotating means being arranged between the separating/combining birefringent element and the first optical-path controlling birefringent element, second polarization rotating means having a ±45-degree variable Faraday rotator and for changing a polarization direction from a parallel to an orthogonal, the second polarization rotating means being arranged between the second optical-path controlling birefringent element and the combining birefringent element; a ½-wavelength plate, for rotating 90 degrees a polarization direction of a light on a central optical path, arranged between the first optical-path controlling birefringent element and the second optical-path controlling birefringent element; and input ports arranged on a side of the separating birefringent element and output ports arranged on a side of the combining birefringent element of the arrangement.
 4. An optical switch according to claim 3, wherein the first polarization rotating means and the second polarization rotating means comprises a combination of a ±45-degree Faraday rotator, a set of two ½-wavelength plates arranged side by side symmetrically in optical axes on the both optical paths to rotate a polarization direction 45 degrees, and a ½-wavelength plate for rotating 90 degrees a polarization direction of a light on a lower-staged optical path, these being arranged symmetric about a center of the arrangement. 